Actuator device based on an electroactive polymer

ABSTRACT

An active matrix array of electroactive polymer actuators (36) is provided, each electroactive polymer actuator (36) having a switching arrangement (34). The switching arrangement comprises an input which is connected to a respective data line (30) and an output which is connected to a first terminal of the associated electroactive polymer actuator (36), wherein a second terminal of each electroactive polymer actuator is connected to a control line (110). A driver provides drive signals which comprise at least first and second drive levels for application to the data lines and third and fourth drive levels for application to the control line. Use is made of of common electrode driving (i.e. the control line driving) to enable low transistor control voltages to be used to switch larger voltages across the electroactive polymer actuators.

FIELD OF THE INVENTION

This invention relates to actuator devices which make use ofelectroactive polymers.

BACKGROUND OF THE INVENTION

Electroactive polymers (EAP) are an emerging class of materials withinthe field of electrically responsive materials. EAP's can work assensors or actuators and can easily be manufactured into various shapesallowing easy integration into a large variety of systems.

Materials have been developed with characteristics such as actuationstress and strain which have improved significantly over the last tenyears. Technology risks have been reduced to acceptable levels forproduct development so that EAPs are commercially and technicallybecoming of increasing interest. Advantages of EAPs include low power,small form factor, flexibility, noiseless operation, accuracy, thepossibility of high resolution, fast response times, and cyclicactuation.

The improved performance and particular advantages of EAP material giverise to applicability to new applications.

An EAP device can be used in any application in which a small amount ofmovement of a component or feature is desired, based on electricactuation. Similarly, the technology can be used for sensing smallmovements.

The use of EAPs enables functions which were not possible before, oroffers a big advantage over common sensor/actuator solutions, due to thecombination of a relatively large deformation and force in a smallvolume or thin form factor, compared to common actuators. EAPs also givenoiseless operation, accurate electronic control, fast response, and alarge range of possible actuation frequencies, such as 0-20 kHz.

Devices using electroactive polymers can be subdivided into field-drivenand ionic-driven materials.

Examples of field-driven EAPs are dielectric elastomers,electrostrictive polymers (such as PVDF based relaxor polymers orpolyurethanes) and liquid crystal elastomers (LCE).

Examples of ionic-driven EAPs are conjugated polymers, carbon nanotube(CNT) polymer composites and Ionic Polymer Metal Composites (IPMC).

Field-driven EAP's are actuated by an electric field through directelectromechanical coupling, while the actuation mechanism for ionicEAP's involves the diffusion of ions. Both classes have multiple familymembers, each having their own advantages and disadvantages.

FIGS. 1 and 2 show two possible operating modes for an EAP device.

The device comprises an electroactive polymer layer 14 sandwichedbetween electrodes 10, 12 on opposite sides of the electroactive polymerlayer 14.

FIG. 1 shows a device which is not clamped. A voltage is used to causethe electroactive polymer layer to expand in all directions as shown.

FIG. 2 shows a device which is designed so that the expansion arisesonly in one direction. The device is supported by a carrier layer 16. Avoltage is used to cause the electroactive polymer layer to curve orbow.

The nature of this movement for example arises from the interactionbetween the active layer which expands when actuated, and the passivecarrier layer. To obtain the asymmetric curving around an axis as shown,molecular orientation (film stretching) may for example be applied,forcing the movement in one direction.

The expansion in one direction may result from the asymmetry in theelectroactive polymer, or it may result from asymmetry in the propertiesof the carrier layer, or a combination of both.

In certain applications, an array of actuators can be useful, forinstance in positioning systems and controlled topology surfaces.However, as the driving voltages of the actuators are fairly high itquickly becomes expensive to drive each actuator individually with itsown driver IC.

A passive matrix array is a simple implementation of an array drivingsystem using only row (n rows) and column (m columns) connections. Asonly (n+m) drivers are required to address up to (n×m) actuators, thisis a far more cost effective approach—and also saves cost and space ofadditional wiring.

Ideally, in a passive matrix device, each individual actuator should beactuated up to its maximum voltage without influencing the adjacentactuators. However, in arrays of traditional EAP actuators (without anyvoltage threshold behavior) some cross talk to adjacent actuators willbe present. When voltage is applied to actuate one actuator, theactuators around it also experience a voltage and will partiallyactuate, which is an unwanted effect for many applications.

This situation is for example described in U.S. Pat. No. 8,552,846,which discloses a passive matrix driving of EAPs without a thresholdvoltage or bistability. In the approach disclosed, a best actuationcontrast ratio of 3:1 is achieved (i.e. “non-actuated” actuators show33% of maximum actuation). This gives a 9:1 contrast ratio to thepressure level since the applied pressure scales with V². This approachalso only works for 2 level driving.

Hence, with a passive matrix addressing scheme it is not straightforwardto individually address each actuator independently of the others.

The use of an active matrix for addressing arrays of electroactivepolymer actuators has been contemplated, for example for electronicbraille applications. An active matrix approach involves providing aswitching device at each electroactive polymer actuator, at theintersection of a row conductor and a column conductor. In this way,each actuator in the array can—if desired—be individually actuated. Anactive matrix addressing scheme means it is possible to have any randompattern of actuators in the array actuated at the same time.

SUMMARY OF THE INVENTION

A problem arises that the switching device, for example transistor,needs to be able to withstand the high actuation voltages required todrive the electroactive polymer actuators. Many EAP designs haveactuation voltages of hundreds of volts, which is far above the possiblevoltages which can be handled by existing transistors suitable forintegration into an array device. Thus, a conventional active matrixaddressing scheme is only suitable for driving electroactive polymeractuators with particularly low actuation voltages, for example up toaround 40V if thin film transistors are used as the switching elements.Above this voltage there will be leakage across the switching TFTdriving transistor. Polysilicon transistors will have lower voltagelimits, for example 20V.

There is therefore a need for an active matrix addressing scheme whichenables relatively low voltage switching devices to be used foractuating relative high voltage electroactive polymer actuators.

It is an object of the current invention to fulfill the aforementionedneed at least partially. This object is achieved at least partially bythe invention as defined by the independent claims. The dependent claimsprovide advantageous embodiments.

According to examples in accordance with an aspect of the invention,there is provided an actuator device comprising:

an active matrix array comprising a plurality of rows and columns ofelectroactive polymer actuators, a set of data lines and a set ofaddressing lines;

a respective switching arrangement associated with each electroactivepolymer actuator, wherein the switching arrangement comprises an inputwhich is connected to a respective data line and an output which isconnected to a first terminal of the associated electroactive polymeractuator, wherein a second terminal of each electroactive polymeractuator is connected to a control line; and

a driver for providing drive signals which comprise at least first andsecond drive levels for application to the data lines and third andfourth drive levels for application to the control line.

This device makes use of common electrode driving to enable lowswitching arrangement (e.g. transistor) control voltages to be used toswitch larger voltages across the electroactive polymer actuators.

The switching arrangement is for switching a voltage on the data line tothe associated electroactive polymer actuator.

The array may comprise a plurality of sub-arrays and there is a sharedcontrol line for each sub-array, wherein each sub-array for examplecomprises a row of electroactive polymer actuators. Alternatively, theremay be a shared control line all of the electroactive polymer actuators.

The electroactive polymer actuators may have a threshold voltage belowwhich there is minimal actuation and a maximum drive voltage at whichthere is full actuation. The use of threshold voltage behavior avoidscross talk between actuators so that individual actuators may beaddressed.

In one example, the first and third drive levels comprise 0V, and thefourth drive level comprises a voltage which is greater than the voltageof the second drive level. The first and third drive levels reset theactuators to 0V. The fourth drive level provides actuation of theactuators, but using the opposite terminal to the terminal controlled bythe switching arrangement. Thus, the switching arrangement is notexposed to the fourth drive level voltage.

The voltage of the fourth drive level may then be the maximum drivevoltage, and the difference between the voltages of the second andfourth drive levels is equal to or less than the threshold voltage. Thefourth drive level initiates actuation, but when the second drive levelis applied to the data line, an actuator may be turned off before it hasbeen physically (rather than electrically) actuated.

Alternatively, the voltage of the fourth drive level may be a negativevoltage of magnitude equal to or less than the threshold voltage and thevoltage of the second drive level is a positive voltage such thatdifference between the voltages of the second and fourth drive levels isequal to the maximum drive voltage. In this case, the fourth drive levelis used to turn off the actuators, because fourth drive level alone(i.e. when there is 0V on the data line) is not sufficient to actuatethe actuator. In this case, the actuators are not electrically actuateduntil they are addressed.

The switching arrangement for example comprises a transistor. It maycomprise a thin film transistor, for example a polysilicon transistor, alow temperature polysilicon transistor or even an amorphous silicontransistor. It may also be a semiconductor oxide transistor such asIndium Gallium Zinc Oxide or related types of oxides as known in theart.

By way of example, maximum source-drain voltage may be less than 50V,for example less than 40V and possibly even less than 25V, whereas themaximum voltage to be provided across the electroactive polymer deviceis more than 50V, for example more than 60V, possibly more than 70V andpossibly more than 80V. The gate source and gate drain voltages may alsoeach be limited to the voltage levels listed above, so that in aparticular design, the maximum voltage may apply to all three of thegate source, gate drain and drain source voltages.

Examples in accordance with another aspect of the invention provide amethod of actuating a device which comprises an active matrix array ofrows and columns of electroactive polymer actuators, each electroactivepolymer actuator having a switching arrangement associated with eachelectroactive polymer actuator, wherein the switching arrangementcomprises an input which is connected to a respective data line and anoutput which is connected to a first terminal of the associatedelectroactive polymer actuator, wherein a second terminal of eachelectroactive polymer actuator is connected to a control line, whereinthe method comprises:

setting all the electroactive polymers to a non-actuated state using athird drive level provided on the associated control lines;

driving all the electroactive polymer actuators towards a first state byproviding a first drive level on the associated data lines and a fourthdrive level on the associated control lines;

before the electroactive polymer actuators reach the first state drivingselected electroactive polymer actuators to a second state by applying asecond drive level on the associate data lines.

This method makes use of common electrode driving to enable lowtransistor control voltages to be used to switch larger voltages acrossthe electroactive polymer actuators.

The electroactive polymer actuators may have a threshold voltage belowwhich there is minimal actuation and a maximum drive voltage at whichthere is full actuation.

In one example, the first state is an actuated state and the secondstate is a non-actuated state. For this purpose, the first and thirddrive levels comprise 0V, and the fourth drive level comprises a voltagewhich is greater than the voltage of the second drive level. The voltageof the fourth drive level may be the maximum drive voltage and thedifference between the voltages of the second and fourth drive levels isequal to or less than the threshold voltage. In this way, the fourthdrive level is used to electrically actuate the actuators, and theaddressing then selectively deactivates them.

In another example, the first state is an non-actuated state and thesecond state is an actuated state. The voltage of the fourth drive levelmay be a negative voltage of magnitude equal to or larger than thethreshold voltage and the voltage of the second drive level is apositive voltage such that difference between the voltages of the secondand fourth drive levels is equal to the maximum drive voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a known electroactive polymer device which is not clamped;

FIG. 2 shows a known electroactive polymer device which is constrainedby a backing layer;

FIG. 3 shows a generic active matrix circuit for one electroactivepolymer actuator;

FIG. 4 shows a generic active matrix addressing scheme;

FIG. 5 shows a first example of a high voltage circuit using low voltagetransistors;

FIG. 6 shows a second example of a high voltage circuit using lowvoltage transistors;

FIG. 7 shows a modification to the circuit of FIG. 6 to make it suitablefor driving an electroactive polymer actuator;

FIG. 8 shows a modification to the circuit of FIG. 7 to provide a signalstorage function;

FIG. 9 shows a first example of an electroactive polymer actuatordriving circuit;

FIG. 10 shows a second example of an electroactive polymer actuatordriving circuit;

FIG. 11 shows a third example of an electroactive polymer actuatordriving circuit for implementing a common electrode drive scheme;

FIG. 12 shows a first example of electroactive polymer device with athreshold behavior;

FIG. 13 shows how the device of FIG. 12 alters the displacement-voltagecharacteristic;

FIG. 14 shows a second example of electroactive polymer device with athreshold behavior;

FIG. 15 shows a third example of electroactive polymer device with athreshold behavior;

FIG. 16 shows a fourth example of electroactive polymer device with athreshold behavior;

FIG. 17 shows a fifth example of electroactive polymer device with athreshold behavior;

FIG. 18 shows how the device of FIG. 17 alters the displacement-voltagecharacteristic;

FIG. 19 shows a sixth example of electroactive polymer device with athreshold behavior;

FIG. 20 shows a drive circuit for an electroactive polymer actuatorusing diodes;

FIG. 21 is used to explain the addressing sequence using the circuit ofFIG. 20;

FIG. 22 shows a drive circuit for an electroactive polymer actuatorusing a MIM diode; and

FIG. 23 shows a drive circuit for an electroactive polymer actuatorusing two drive transistors.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides an active matrix array of electroactive polymeractuators, each electroactive polymer actuator having a switchingarrangement.

The actuator comprises an electroactive polymer structure for providinga mechanical actuation such that the structure defines a non-actuatedstate and at least one actuated state (different from the non-actuatedstate) attainable by application of an electrical drive signal to theelectroactive polymer structure. The electroactive polymer structurethereto comprises and EAP material. Such material is capable of inducingor allowing a mechanical deformation upon subjecting it to an electricalsignal that can be provided in the form of an electrical drive signal tothe structure. The actuator or structure can have an electrodearrangement for providing the drive signal to the EAP material. Theelectrode structure can be attached to the EAP material directly or withintermediate layers in between.

The EAP material layer of each unit may be sandwiched between electrodesof the electrode structure. Alternatively, electrodes can be on a sameside of the EAP material. In either case, electrodes can be physicallyattached to the EAP material either directly without any (passive)layers in between, or indirectly with additional (passive) layers inbetween. But this need not always be the case. For relaxor or permanentpiezoelectric or ferroelectric EAPs, direct contact is not necessary. Inthe latter case electrodes in the vicinity of the EAPs suffices as longas the electrodes can provide an electric field to the EAPs, theElectroactive polymer structure will have its actuation function. Theelectrodes may be stretchable so that they follow the deformation of theEAP material layer.

The electrical drive signal can be a voltage signal or a current signaldepending on the EAP material used (see herein below).

The switching arrangement comprises an input which is connected to arespective data line and an output which is connected to a firstterminal of the associated electroactive polymer actuator, wherein asecond terminal of each electroactive polymer actuator is connected to acontrol line. A driver provides drive signals which comprise at leastfirst and second drive levels for application to the data lines andthird and fourth drive levels for application to the control line.

Use is made of of common electrode driving (i.e. the control linedriving) to enable low transistor control voltages to be used to switchlarger voltages across the electroactive polymer actuators.

FIG. 3 shows a generic active matrix circuit for use in an array ofelectroactive polymer actuators. The array is arranged in rows andcolumns with row conductors 30 and column conductors 32. There areplural rows and columns, so that at the minimum there is a 2×2 array.There may be many rows and columns, for example tens or hundreds of rowsand/or columns.

A circuit as in FIG. 3 is at each intersection (i.e. cross over) of therow and column conductors. The circuit comprises a transistor 34 such asa field effect thin film transistor, with its gate connected to the rowconductor 30 and its source connected to the column conductor 32. Thetransistor is turned on by a select pulse on the row conductor, and itthen couples the voltage on the column conductor 32 to the electroactivepolymer actuator 36 and to a storage capacitor 38.

There is an array of m×n electroactive polymer actuators each driven byan active driving circuit as shown in FIG. 3. There are m row conductors(addressing lines) and n column conductors (data lines), where n≥2 andm≥2, for example n≥4 or n≥10 or n≥50 and/or m≥4 or m≥10 or m≥50. Thefirst electrode of the electroactive polymer actuator 36 is a drivingelectrode. The second electrode of the electroactive polymer actuator isconnected to a reference voltage Vref which may be common to many or allof the electroactive polymer actuators in the array. The storagecapacitor 38 in parallel with the electroactive polymer actuator isoptional, and the function is to help maintain the voltage applied tothe electroactive polymer actuator.

The operation of the circuit is to transfer the data voltage to thedriving electrode of the electroactive polymer actuator only when thegate of the selected transistor 34 is addressed, causing the TFT tobecome conductive. After addressing is completed, the transistor becomesinsulating and the voltage is maintained on the electroactive polymeractuator until it either leaks away or until the electroactive polymeractuator is addressed again. In such a manner the circuit operates as asample-and-hold circuit, whereby the (optional) storage capacitor helpsto maintain the voltage applied to the electroactive polymer actuator.

After addressing, the electroactive polymer actuator will deform to anew actuation state depending upon the driving voltage (Vdr) which ispresent between the data electrode and the reference electrode. Notethat the actuation may well take significantly longer than the period ofaddressing (which will typically be much less than 1 msec). Differentlevels of actuation can be realized by applying different drivingvoltages.

Addressing the array proceeds as shown in FIG. 4 which shows an exampleof a 4×4 array with a reference electrode set at Vref=0V. The startingpoint (not shown) is that all electroactive polymer actuators aredischarged and hence in their non-actuated state. Open circles representactuators which are not actuated, filled circles are actuators which areactuated.

All rows are initially addressed with a non-selection voltage (Vns:typically −10V for typical TFT). In this situation, no data can betransferred to the electroactive polymer actuators.

The first row is then addressed with a selection voltage (Vsel typically+30V for a typical TFT). This is shown in FIG. 4A. The other rows arenot selected (Vns). Two columns are driven with a drive voltage Vdr, andthe other two columns are driven with 0V and these voltages aretransferred onto the first electrode of the respective electroactivepolymer actuators. In this situation, the voltage difference across twoof the electroactive polymer actuators is Vdr: these two electroactivepolymer actuators in the row will be in the actuated mode (actuation maytake some time after addressing is finished). The voltage differenceacross the other two electroactive polymer actuators is 0V, wherebythese two electroactive polymer actuators in the row will remain in thenon-actuated mode.

The second row is then addressed with a selection voltage (Vsel) asshown in FIG. 4B. The other rows are not selected (Vns). Again, twocolumns are driven with a drive voltage Vdr, and two columns are drivenwith 0V and these voltages are transferred onto the first electrode ofthe respective electroactive polymer actuators. In this situation, thevoltage difference across two of the electroactive polymer actuators sis Vdr: these two electroactive polymer actuators in the row will be inthe actuated mode (actuation may take some time after addressing isfinished). The voltage difference across the other two electroactivepolymer actuators is 0V, whereby these two electroactive polymeractuators in the row will remain in the non-actuated mode.

By deselecting the first row, the sample-and-hold nature of theaddressing ensures that the electroactive polymer actuators in the firstrow maintain their voltage (especially if a storage capacitor isincluded) and remain in their state of actuation (or proceed towards itif not already reached).

The third row is then addressed with a selection voltage (Vsel) as shownin FIG. 4C. The other rows are not selected (Vns). Now three columns aredriven with a drive voltage Vdr, one column is driven with 0V and thesevoltages are transferred onto the first electrode of the respectiveelectroactive polymer actuators. In this situation, the voltagedifference across three of the electroactive polymer actuators is Vdr:these three electroactive polymer actuators in the row will be in theactuated mode (actuation may take some time after addressing isfinished). The voltage difference across the other electroactive polymeractuator is 0V, whereby this electroactive polymer actuator will remainin the non-actuated mode. By deselecting the second row, thesample-and-hold nature of the addressing again ensures that theelectroactive polymer actuators in this row maintain their voltage.

The fourth row is then addressed with a selection voltage (Vsel) asshown in FIG. 4D. The other rows are not selected (Vns). Only one columnis driven with a drive voltage Vdr, and the other three columns with 0V.The voltage difference across only one of the electroactive polymeractuators is Vdr.

At the end of the addressing phase, all rows can be deselected (Vns) andvoltages can be removed from the columns whereby the electroactivepolymer actuators will remain in their state of actuation until theirvoltage leaks away, at which point the array may be re-addressed asdescribed above.

It is possible to address several rows of electroactive polymeractuators at the same time, whereby the addressing will proceed evenmore quickly. This is achieved by applying an addressing voltage to morethan one row of electroactive polymer actuators at the same time. Thisis possible if the same pattern of data is to be applied.

In the above example only a two level data driver is considered (0V andVdr). This will result in the lowest cost driver IC's. However infurther examples it may be preferred to also partially actuate theelectroactive polymer actuators. To enable this, a data driver withmultiple data voltages up to Vdr may be used.

Furthermore, whilst in the example above a voltage of one polarity isapplied to the electroactive polymer actuators, in further embodimentsit may be preferred to invert the polarity of the voltage across theelectroactive polymer actuators at regular intervals whereby theperformance of the electroactive polymer actuator will deteriorate lessthan if inversion is not used. This can be achieved by example bychanging the voltage on the reference electrode in a further round ofaddressing and adapting the driving voltages accordingly.

The basic active matrix addressing scheme described above requirestransistors to be able to withstand the same voltages as those which areused to drive the electroactive polymer actuators.

This invention relates to the use of switching devices, such as lowvoltage poly-silicon transistors, which operate at lower voltages thanthe drive voltages of the electroactive polymer actuators.

High voltage heating techniques are known which make use of transistorcontrol switches which operate at lower voltages than required by theheater.

FIG. 5 shows a first example of a drive circuit for a 60V heater. FIG.5(a) shows the heater turned off, and FIG. 5(b) shows the heater turnedon.

The transistors used in the circuit are 20V TFTs which is for examplethe voltage limit for a low temperature polysilicon (LTPS) TFT.

The circuit comprises a pull up resistor 50 connected to high voltage60V line Vddout, and a pull down circuit which comprises three n-typetransistors N1 to N3 in series. The bottom transistor N1 has a controlvoltage applied, which switches between 0 and 20V. The second transistorN2 has a constant bias voltage applied. A bias circuit 52 controls thevoltage applied to transistor N3 and it includes two p-type transistorsP1 and P2. The first p-type transistor P1 is between the gates of thesecond and third n-type transistors N2, N3 with its gate connected tothe node between the transistors N2, N3. When P1 is closed, the biasvoltage is applied to the gates of both transistors N2, N3 so they areboth closed. The second p-type transistor P2 is between the gate andsource of the third n-type transistors N3 with its gate connected to thegate of the second n-type transistor N2. When P2 is closed, there is nosource-drain voltage for transistor N3 so transistor N3 is open.

FIG. 5A shows the voltages in the circuit when the input control voltageis high (20V), in which case P1 is turned on and P2 is turned off. Thismeans that N1-N3 all have a gate voltage of 20V and the output voltageis pulled down to ground (0V) as are the nodes between N1 and N2 andbetween N2 and N3.

FIG. 5B shows the voltages in the circuit after the input controlvoltage is switched low (0V), in which case N1 turns off and the outputvoltage is pulled high. The circuit as a whole functions as an opencircuit because there is no connection to ground as a result of N1 beingturned off. In the absence of the pull up resistor 50 and itsconnections, the output would be floating.

In turn, the node between N2 and N3 is pulled high, which causes P1 toturn off and P2 to turn on. From symmetry, the steady state voltagebetween N2 and N3 will settle at 40V and the steady state voltagebetween N2 and N1 will settle at 20V, in which case the leakage currentsthrough transistors N1 to N3 are all equal.

FIG. 6 shows an example of a drive circuit for an 80V heater. The pulldown circuit comprises four n-type transistors N1 to N4 in series andthe bias circuit 54 controls the voltage applied to transistors N3 andN4 and it includes four p-type transistors P1 to P4. The bias circuitcomprises two of the bias circuits 52 of FIG. 5 stacked one above theother. Again, either all the n-type transistors are turned on to pullthe output low, or else the bottom transistor is turned off and thevoltage drop is shared across each n-type transistor.

FIG. 6A shows the voltages in the circuit when the input control voltageis high (20V), and FIG. 6B shows the voltages in the circuit after theinput control voltage is switched low (0V),

When using these circuits for heating, a high voltage is dropped acrossthe resistor and it is assumed that the resistor can sustain the highvoltage, which is usually the case. However for an electroactive polymerdevice, the device has a capacitive equivalent circuit.

The approach then may be to use the circuit of FIG. 7, in which the loadis the EAP actuator, which is now in addition to the pull up resistor.

The problem of a circuit such as the one shown in FIG. 7 is that thereis static power consumption when the pull down circuit is in the onstate i.e. when one or both terminals of the EAP actuator is pulled downto 0V. Also the circuit has no storage of the drive signal.

The storage issue is easily resolved as is shown in FIG. 8.

An addressing transistor 80 is provided, which enables a data voltageVdata which is for example on a column line to be applied to a localstorage capacitor 82. The gate of the addressing transistor 80 iscoupled to an addressing line 81 to which an addressing voltage Vaddr isprovided. The addressing line is for example a row conductor.

The electroactive polymer actuator can be driven between the supply at80V in this example and 0V with a low voltage addressing scheme, whereVaddr (for example in the range 0 to 20V) is the standard row addresssignals of a matrix array and Vdata is also for example in the range 0to 20V.

The issue of static power consumption remains, and the circuit can onlyachieve two voltage levels across the electroactive polymer device.

The invention provides a circuit approach which resolves one or more ofthese issues, a first example of which is shown in FIG. 9.

The circuit comprises a first circuit 90 as shown in FIG. 8, and asecond circuit 92 as shown in FIG. 8, but sharing a common pull upresistor 50 which connects to one terminal of the electroactive polymeractuator (defined as the first terminal). The overall circuit thus hastwo row addressing lines 94, 96 and two column data lines 98, 100.

The output of the first circuit 90 is connected to one side of theelectroactive polymer actuator 36, and the output of the second circuit92 is connected to the other side of the electroactive polymer actuator36. In this way, the overall drive circuit has access to both sides ofthe electroactive polymer actuator 36 and as a result it becomespossible to overcome problem of static power consumption.

By addressing with low voltage signals Vaddr1,2 and Vdata1,2 drivingwith two levels is made possible without static power consumption.

By providing Vdata1 and Vdata2 high (when the respective row addresssignals are on), both sides of the electroactive polymer actuator willbe coupled to ground. Vdata2 can then be driven to a low value (whilstVaddr2 is still on). 0V will remain across the electroactive polymeractuator but the second terminal will be floating so that no current canflow through the electroactive polymer actuator itself

Taking Vdata1 low as well (whilst Vaddr1 is still on) will not only shutoff the current flow through the electroactive polymer actuator but willalso prevent current consumption from the main supply. The electroactivepolymer actuator will have 0V stored, but no current will flow throughthe electroactive polymer actuator and it will float up to the highsupply (on both terminals).

Note that only the first circuit 90 needs to be switched to the highimpedance state to achieve a high voltage across the actuator, whereasboth circuits 90, 92 are held in the high impedance state for a low(i.e. zero) voltage to be maintained across the actuator. Therefore whenaddressing with Vaddr1 and Vaddr2 to achieve a high voltage on theactuator, Vdata1 should start high and transition to low within theaddress period. For a low voltage, both Vdata1 and Vdata2 start high andtransition to a low value during the address period.

To address the electroactive polymer actuator with a high voltage, amodified drive scheme is provided.

Setting Vdata1 low (so that the circuit 90 is an open circuit) andVdata2 high (so that the circuit 92 is grounding its terminal of theelectroactive polymer actuator) when the respective row address signalsare on, will cause the drive voltage Vddout (e.g. 80V) to be providedacross the electroactive polymer actuator.

These voltages can then be held to cause the 80V to be stored with nocurrent drawn after the address signal go low.

The circuit 92 can then be driven to the open circuit state. This willprevent current flow through the actuator since one terminal isfloating. This again prevents static power consumption.

FIG. 10 shows a modification to the circuit of FIG. 9, in which eachcircuit 90, 92 has its own respective pull up device 50, 100, so thatthere is one pull up device connected to each terminal of theelectroactive polymer actuator 36.

This enables a three level drive scheme to be achieved.

By way of example, voltage levels 80V, 0V, and −80V can be providedacross the electroactive polymer device.

This circuit arrangement may therefore be used to provide a high voltageinversion driving scheme. It has previously been observed that suchinversion driving improves both the amplitude of the actuation andreduces and drift in actuation with prolonged usage.

The addressing can proceed in the following manner.

(i) To drive to +80V

Vaddr1,2 are driven high, Vdata1 is high (circuit 90 is grounded) andVdata2 is low (circuit 92 is open circuit). The electroactive polymeractuator is thus charged to 80V (i.e. the terminal connected to thesecond circuit 92 is at a higher voltage than the terminal connected tothe first circuit 90). Then Vaddr1,2 go low. The capacitors 82 maintainthe same voltages.

(ii) To drive to 0V

Vaddr1,2 are driven high. Vdata1,2 are initially high so that bothcircuits are grounded. They are then made low so that both terminals ofthe electroactive polymer actuator are at the same voltage but there isno static power consumption because the electroactive polymer actuatorterminals float up to the power supply voltage. Vaddr1,2 then go low.

The timing of operation of the circuits 90 and 92 is synchronized.

(ii) To drive to −80V

Vaddr1,2 are driven high. Vdata1 is low (circuit 90 is open circuit) andVdata2 is high (circuit 92 is grounded). The electroactive polymeractuator is thus charged to −80V (i.e. the terminal connected to thesecond circuit 92 is at a lower voltage than the terminal connected tothe first circuit 90). Then Vaddr1,2 go low. The capacitors 82 maintainthe same voltages.

If the two supply voltages that are shown at 80V in FIG. 10 are changedto different levels (each 80V or less) then different sets of voltagesacross the electroactive polymer actuator can be derived e.g. suppliesof 80V and 60V would achieve 80V, 0V and −60V on the electroactivepolymer actuator.

The approach described above realizes a limited number of drivinglevels. However, intermediate actuation levels may be obtained byapplying pulse width modulation (PWM) schemes. A PWM approach may forexample help for some types of device, particularly in order to maintaina steady state.

The examples above make use of a series of pull down transistors toenable low voltage transistors to be used. Another approach is to makeuse of counter electrode driving.

FIG. 11 shows a basic active matrix switching circuit comprising a rowconductor 30, column conductor 32, transistor 34 and the electroactivepolymer actuator 36. One terminal of the electroactive polymer actuatoris connected to the transistor 34 and the other is connected to a commoncounter electrode 110 to which a counter electrode voltage Vce isapplied. A storage capacitor 38 may again be provided. The transistormay be a thin film transistor, and it may be polysilicon or amorphoussilicon.

The counter electrode 110 may be common to a sub-set of theelectroactive polymer actuators, for example all electroactive polymeractuators in an addressing row, or it may be common to all electroactivepolymer actuators in the array. Traditional driving of such an activematrix circuit limits the driving voltage to around 40 V.

A high driving voltage method for the active matrix array may beachieved by driving the array from both the counter electrode connectionand from the data driver. In this manner, a high driving voltage may beprovided to selected electroactive polymer actuators in the arraywithout introducing a high voltage to the driving electrode. Thisensures that the addressing transistor 34 is not subjected to highvoltages, since the high voltage is only present on the counterelectrode. As a result, it will not leak, age or fail completely.

The counter electrode may be set to a different (second) non-zerovoltage before applying the driving voltages from the data driver. Inthis manner, a higher voltage across the electroactive polymer actuatormay be achieved with a data driver of a given voltage.

In particular, the actuation voltage is equal to the drive voltage lessthe counter electrode voltage. In this case, it is beneficial if theelectroactive polymer actuator has a threshold voltage which is at leastas high as the second counter electrode voltage to avoid that allelectroactive polymer actuators are actuated. Ways to implementthreshold voltages are discussed below.

In a first example of the use of the circuit of FIG. 11, theelectroactive polymer actuators are assumed to have a threshold voltageof 30V and an actuation voltage of 60V, which exceeds the normal rangeof the active matrix addressing. The threshold voltage represents anactuation level below which there is greatly reduced actuation comparedto the actuation above the threshold. The high voltage driving proceedsas follows, assuming the electroactive polymer actuators to be in aninitial, non-actuated state.

All data drivers in the array are set to a reference voltage e.g. 0V.

The counter electrode voltage Vce is 0V at this time. All addressingtransistors 34 are then are driven to the addressing (conducting) statee.g. with their gate electrodes at 40V. This ensures that the voltage onthe driving electrode of each electroactive polymer actuator remains at0V by connection to the data driver voltages present on the columnconductors 32.

The counter electrode voltage Vce (for example +60V) to be applied toall the electroactive polymer actuators is driven onto the commoncounter electrode 110. When this voltage is applied, a current flowsinto the device to charge up the capacitance until the counter electrodereaches the applied voltage and there is 60V across the electroactivepolymer actuator. At this point, due to the slow response speed, allelectroactive polymer actuators are in the same physically non-actuatedstate whilst the charge on the electroactive polymer actuators isconsistent with the actuated state. As a result, all electroactivepolymer actuators will begin to actuate unless they are de-activated inthe addressing phase.

In this addressing phase, all addressing transistors 34 are first drivento the non-addressing (insulating) state e.g. with their gate electrodesat −5V.

The data is then applied to the array one line at a time in the normalmanner (by addressing one line of addressing transistors at a time). Inthis case, the actuation data will be 0V, as this will cause thoseelectroactive polymer actuators which must switch position to do so. Byapplying 0V to the electroactive polymer actuator, there will be 60Vacross the electroactive polymer actuator. A data voltage of 30V willinstead result in a voltage of +30V across the device which will bebelow the threshold and the electroactive polymer actuator will eithernot actuate (if addressing is carried out immediately) or will return tothe non-actuated state (if there is a delay before addressing).

Thus, the electroactive polymer actuators are all driven electrically totheir actuated state. This state is either maintained after addressing,or it is reversed before the actuators have had time to respondphysically.

In this example, the data line voltages comprise a first drive levelwhich is 0V and a second drive level which is 30V.

The common electrode voltages comprise a third drive level which is 0V,and a fourth drive level which is 60V. More generally, the voltage ofthe fourth drive level is greater than the voltage of the second drivelevel.

The voltage of the fourth drive level (60V) is the maximum drive voltageso that when it is applied, the actuator is electrically driven to itsmaximum actuation state (when the data voltage is zero). The differencebetween the voltages of the second and fourth drive levels (60V-30V=30Vin this example) is equal to or less than the threshold voltage so thatwhen the data voltage is set at the second drive level (30V) there isde-addressing.

The addressing transistors are made conducting during the period whenthe counter electrode voltage switches, as otherwise the voltage on thedriving electrode will also switch to the counter electrode voltage (asno current can flow away) and the addressing transistor will be damaged.

The common electrode voltage can remain at the high level. To turn offthe device as a whole, all data lines may be driven to 0V to turn offthe transistors, and then the common electrode voltage can be drivendown to 0V in a controlled manner.

In a second implementation the electroactive polymer actuators areassumed to have a threshold voltage of 60V and an actuation voltage of90V—which exceeds the normal range of the active matrix addressing. Thehigh voltage driving proceeds as follows, assuming the electroactivepolymer actuators to be in an initial, non-actuated state.

All data drivers in the array are set to a reference voltage e.g. 0V.

The counter electrode voltage Vce is 0V at this time. All addressingtransistors are driven to the addressing (conducting) state e.g. withtheir gate electrodes at 40V. This ensures that the voltage on thedriving electrode remains at 0V.

The counter electrode voltage Vce (−60V) to be applied to all theelectroactive polymer actuators is driven onto the common counterelectrode. When this voltage is applied, a current flows into the deviceto charge up the device capacitance until the counter electrode reachesthe applied voltage (there is 60V across the electroactive polymeractuator). At this point, all electroactive polymer actuators are in thesame state (i.e. non-actuated) with a charge on the electroactivepolymer actuators which is consistent with this actuation state.

All addressing transistors are then driven to the non-addressing(insulating) state e.g. with their gate electrodes at −5V.

The data is applied to the array one line at a time in the normal manner(by addressing one line of addressing transistors at a time). In thiscase, the actuation data will be 30V, as this will cause thoseelectroactive polymer actuators which must switch position to do so (90Vacross the device). A data voltage of 0V will result in a voltage of 60Vacross the device, which will be below the threshold and the device willnot actuate.

This method differs from the first in that the initial driving is to thenon-actuated state, and the row-by-row addressing is to switch theelectroactive polymer actuators to the addressed state.

In this example, the data line voltages comprise a first drive levelwhich is 0V and a second drive level which is 30V.

The voltage of the fourth drive level (-60V) is a negative voltage ofmagnitude equal to or less than the threshold voltage (60V in thisexample) and the voltage of the second drive level is a positive voltage(30V in this example) such that difference between the voltages of thesecond and fourth drive levels is equal to the maximum drive voltage(30V13 60V=90V). The fourth drive level alone is not sufficient toactuate the actuator.

The addressing transistors are again conducting during the period whenthe counter electrode voltage switches, as otherwise the voltage on thedriving electrode will also switch to the counter electrode voltage (asno current can flow away) and the addressing transistor will be damaged.

The operation of the circuit is to transfer the data voltage to thedriving electrode of the electroactive polymer actuator only when thegate of the selected TFT is addressed, causing the TFT to becomeconductive. After addressing is completed, the TFT becomes insulatingand the voltage is maintained on the device until it either leaks awayor until the device is addressed again. In such a manner the circuitoperates as a sample-and-hold circuit, whereby the (optional) storagecapacitor helps to maintain the voltage applied to the device.

After addressing, the device will deform to a new actuation statedepending upon the driving voltage (Vdr) which is present between thedata electrode and the reference electrode. The actuation may well takesignificantly longer than the period of addressing, which will typicallybe much less than 1 msec. Different levels of actuation can be realizedby applying different driving voltages.

As will be clear from the description above, some designs may make useof a threshold behavior of the device. The electroactive polymeractuator does not inherently exhibit threshold behavior. Some ways tocreate a structure with the desired threshold behavior will now bediscussed.

An artificially created threshold may be created, to avoid unwantedactuation effects up to this threshold, using either mechanical effectsor electrical (driving signal) effects, or combinations of these.

Mechanical threshold effects may for example be implemented usinggeometry, mechanical clamping, or surface “stickiness”. Electricalthreshold effects may for example be implemented using electrostaticattraction or electrical breakdown behavior. A combination of theseeffects may also be used to efficiently implement a voltage threshold.

This threshold may be considered to be a delay, in that physicalactuation is delayed until a certain drive level is reached.

FIG. 12 shows a first example based on a geometric effect, using amechanical structure to implement the delay.

The device comprises an electroactive polymer layer 120 within a chamber122. The chamber has a lid 124 suspended over the electroactive polymerlayer 120. The lid is seated on a rim which means it is suspended overthe EAP layer. Driving of the electroactive polymer layer with a firstrange of applied drive signals raises it towards the lid. After contacthas been made (at the maximum drive signal within the first range),further actuation causes the lid to raise as shown in the lower image.Thus, there is a range of input drive signals which only cause movementof the electroactive polymer layer within the gap beneath the lid. Whenthe maximum drive signal in this range is reached, contact is made. Thiscorresponds to the threshold voltage of the overall device. Above thisdrive signal, further driving in a second range provides progressinglifting of the lid, which corresponds to the mechanical output of thedevice.

Thus, a partially actuated element will not displace the lid, but afully actuated actuator will give displacement, though the penalty is alimited full displacement of the actuated surface.

As shown FIG. 13, the effect of the delay is to lower the displacementcurve so that there is no displacement until a threshold V_(T) isreached. This has the effect of lowering the maximum displacement.

The actuator can provide more displacement if it is clamped using aretainer system, for example a snap system to create a threshold voltagefor actuation. This threshold voltage then corresponds to a requiredforce to overcome the retainer function.

FIG. 14 shows an example having a retaining mechanism 140 in the form ofsnap hooks which the lid 124 must pass before displacing. The snap hooksrequire a threshold force to be applied to the lid before it can movepast the hooks. The corresponding displacement versus voltagecharacteristic (plot 142) as well as force versus voltage characteristic(plot 144) are shown.

After the snap-through the actuator will keep increasing itsdisplacement with more applied voltage. When the voltage is removed, thesystem returns to its initial flat state. The snap hooks may allow freepassage in the downward direction of the lid, or else the device mayneed to be reset by an additional applied force.

In a further mechanical embodiment, the threshold voltage may be inducedby adding a defined “stickiness” between the electroactive polymerstructure (i.e. the polymer layer and its own substrate) and a supportstructure. The stickiness can only be overcome by increasing the voltageacross the electroactive polymer layer until its force overcomes thestickiness of the system.

The stickiness could be implemented by

chemical modification of the surfaces (applying a glue-like property),

introducing a fluid between the surfaces (using capillary forces),

mechanical/topological modification of the surface, for example a“Velcro” like structure.

The examples above make use of a delay mechanism based on a mechanicalstructure, which for example defines the output of the device. Analternative is based on an electrostatic effect as shown in FIG. 15.

The actuator has an additional electrode 150 on the surface below theelectroactive polymer structure. The electrostatic attraction betweenone electrode of the electroactive polymer layer 120 and the extraelectrode 150 on the surface creates a restrictive force whichconstrains bending.

If the electrostatic force is overcome by the bending force, theactuator will bend. This reduces the electrostatic force drastically, asthe force is a function of the separation between the electrodes (d)squared. Any bending will increase d and the electrostatic force isreduced, leading to further bending and hence more reduction ofF_electrostatic, and the threshold is overcome.

The graph shows the corresponding displacement versus voltagecharacteristic (plot 152), the force versus voltage characteristic (plot154) and the electrostatic force versus voltage characteristic (plot156).

An advantage of this system is that the electrostatic force is almostinstantaneous and the electroactive polymer layer force is slow torespond, which is favorable for keeping the actuator tightly clamped atlower voltages. A dynamic effect can be realized by exploiting thedifference in capacitance between the electroactive polymer layer andthe substrate. In this configuration, the electrostatic force will workto constrain the EAP device as soon as a voltage is applied. Theelectroactive polymer actuator will however slowly build up to itsmaximum force from a step voltage input. This can cause a delayedthreshold effect. Thus, when a step voltage is applied, theelectrostatic force first holds the device down until the actuationforce overcomes the electrostatic force threshold and pops up to give adisplacement.

The threshold value is thus determined partly by the geometry of theactuator and partly by the speed of actuation.

Another possible implementation of a delay mechanism to provide athreshold comprises an electrical component which implements a thresholdvoltage or a breakover voltage for controlling the application of theapplied drive signal to the electroactive polymer layer.

FIG. 16 shows an example, in which the electroactive polymer layer 120is connected electrically in series with an electrical threshold orbreakover element 160 shown as a DIAC (diode AC switch). Other thresholdelements may be used such as a Shockley diode, silicon controlledrectifier or other thyristor. This element may be part of theelectroactive polymer structure, for example as organic semiconductinglayers (in p-n-p-n sequence) as a part of the substrate stack.Alternatively for larger actuators in an array, the element can be asurface mount device component in series connection with each actuator.

For an applied voltage below the breakover or threshold voltage, thereis no deformation induced as the voltage drop arises across thethreshold or breakover element. For a larger applied voltage, theelectroactive polymer layer will deform.

Another possible implementation for the delay mechanism comprises asecond electroactive polymer structure, wherein the second electroactivepolymer structure comprises an electrode for receiving the applied drivesignal to the device, wherein upon deformation of the secondelectroactive polymer structure by a predetermined amount, the applieddrive signal is coupled to the (main) electroactive polymer structure.

FIG. 17 shows an example. The overall device comprises a main actuator170 and a subsidiary actuator 172. The subsidiary actuator is smallerthan the main actuator and it defines a control part which is a non-loadbearing device.

The use of two sequential actuators enables a threshold to beimplemented. The subsidiary actuator acts as a mechanical switch whilethe main actuator is the functional actuator. When the voltage is belowthe threshold voltage the switch is off, as shown for voltages V=0 andV=V1 in FIGS. 17A and 17B.

At and above the threshold voltage, for example V=V2 as shown in FIG.17C, the switch is on and the functional actuator is at once fullypowered to that voltage.

The contact between the two actuators provides contact of their drivingelectrodes, so that the subsidiary actuator delays the application ofthe drive voltage to the main actuator.

FIG. 18 shows the displacement function for the main actuator, and itcan be seen that there is an abrupt cutoff of the displacement function.

The sequential ordering can be configured in several different ways withdifferent actuator configurations and switching actuator geometries. Thecontact can be made by the electrode of the electroactive polymerstructure or by an additional contact pad made on the back side of thesubstrate, depending on the actuator geometry.

As mentioned above, another way to implement the delay function is byintroducing a sticking property.

FIG. 19 shows an implementation in which the expansion of theelectroactive polymer layer 120 is constrained to be in-plane.

This design could be based on a free standing device (as in FIG. 1). Forexample, the two layers may be fixed at one side and otherwise be freeto expand in all directions.

The layer is provided against a substrate 192 and there is frictionalresistance between them which resists the relative sliding movementuntil the frictional force is overcome.

In this way, the friction functions as the delay mechanism, anddetermines the threshold.

In order to drive the device in a way which overcomes the friction, anac drive scheme may be used. For example a controller 194 is used toapply a high frequency ac ripple added to a dc driving signal to enablerelative slippage when the actuator moves from one position to a nextposition. The next position can also be held by removal of the appliedvoltage due to the friction, so that a bistable effect is obtained.

As shown in the voltage time profile in FIG. 19, the driving of thedevice starts with an ac voltage with only a small dc offset.Electroactive polymers actuate symmetrically for positive and negativevoltages, so that there will result a vibration around the non-actuatedstate. This will result in a reduction of friction and prepare theelectroactive polymer layer for a smooth actuation movement, whichoccurs as soon as the driving voltage increases.

The electroactive polymer layer then continues to deform during the nexttime period depicted in the graph, where there are active vibrationsduring the deformation (induced by the ac component superimposed on therising dc voltage level).

Finally, following a short period where the ac signal is superimposedupon an essentially constant dc level, to allow for any delay in themovement of the electroactive polymer layer in reaching its final state,the voltage is removed which, if the residual friction is sufficient,will result in a second stationary state being retained. Subsequentlythe device can be reset by applying only a small ac signal to overcomethe friction and bring the device back to its original state. Hence thedevice has multiple arbitrary stable states with a reset possibility. Inthis embodiment, it may be advantageous to reduce the ac signalamplitude slowly to allow the device to settle into its most stable(highest friction) state.

The various examples described above essentially provide an actuatordevice which has a threshold function.

The circuits above are based on the use of transistors. Whilst theMOSFET properties of amorphous silicon TFTs allow for devices to bedriven to both higher voltages and lower voltages through the sameswitching TFT, there are other—lower cost—active elements such as diodeswhere this is not the case.

In principle it is possible to address an EAP device through a diode inseries at the crossing point of rows and columns. However as the diodeonly conducts in one direction, reducing of the activation state of eachdevice depends on the self-discharging function of an EAP device. Thismay result in long not-wanted on-times of pixels of an array.

FIG. 20 shows a switching arrangement which makes use of diodes. A firstdiode 200 is between a first addressing line 202 and a first terminal ofthe electroactive polymer actuator 36, and a second diode 204 is betweenthe first terminal of the electroactive polymer actuator and a secondaddressing line 206. The diodes are thus in series between the twoaddressing lines 202, 206 with the same polarity. A second terminal ofthe electroactive polymer actuator is connected to a select line 208.The first and second addressing lines comprise column conductors and theselect line comprises a row conductor.

There is thus a first diode between a first addressing line and a firstterminal of the electroactive polymer actuator, and a second diodebetween the first terminal of the electroactive polymer actuator and asecond addressing line, wherein a second terminal of the electroactivepolymer actuator is connected to a select line. This arrangement usestwo diodes; one for addressing and the other for de-addressing.

The first diode 200 is the addressing diode for charging theelectroactive polymer actuator from the first addressing line, and thesecond diode 204 is the de-addressing diode for discharging theelectroactive polymer actuator to the second addressing line (i.e. driveit to lower voltages).

In order to limit the on-time, as may be required depending on theapplication requirements, this circuit provides active deactivation orde-addressing of the electroactive polymer actuator. It allows for arapid discharge of the electroactive polymer actuator (and if presentits storing capacitor).

The addressing scheme is extended by a corresponding de-addressingscheme as explained with reference to FIG. 21 which shows a 4×4 matrix.

At each crossing point the circuit of FIG. 20 is provided. There are nowtwo columns conductors 202, 206 for each column of actuators so thatthere are 2m columns and n rows.

During each cycle, the rows are addressed in turn. When one row is beingaddressed, the electroactive polymer actuators in the row are eitheraddressed or actively de-addressed.

FIG. 21 shows two parts of an addressing cycle in which the first row isselected.

FIG. 21A shows all electroactive polymer actuators in the first rowbeing actuated.

During addressing, the second column conductors 206 are held high (Vh)so that the actuators cannot discharge. The first column conductors 202of the selected columns are also brought high (Vh) to charge theelectroactive polymer actuators in those columns. In the example of FIG.21A all four actuators are addressed.

During de-addressing as shown in FIG. 21B, the first data columns 202are held low (V1) so that in combination with the addressing linevoltage the first diode 200 is not conducting. The second diodes 204 ofthe selected columns are conducting as a result of a low dischargevoltage (Vd) applied to the selected second column conductors 206. Theother second column conductors have a high voltage applied (Vh) so thatthe diodes 204 do not conduct.

The row conductor for the selected row has a selection voltage applied(Vsel) and all other rows have a non-selection voltage applied (Vns). Aswill be explained below, a different non-selection voltage is usedduring the charging and discharging stages. Only if the de-addressingrow is activated AND the corresponding second data column 206 isswitched to the de-activation voltage (Vd), the actuator at the crossingpoint will be deactivated (discharged). The low voltage Vd is notsufficient alone to forward bias the diode 204 when the lownon-selection voltage Vns is applied to the row conductor.

The addressing and de-addressing operations are sequential. They cannotbe at the same time as this would provide a short between the two columnconductors.

By way of example, for an actuation voltage of 200V, the followingvoltage levels are possible: Vh=100V, V1=−100V, Vd=−100V and Vns=+100Vor −100V depending on the stage of the driving sequence.

One possible driving sequence is:

1. Charging

The address line 208 for the row to be addressed is driven to Vsel=−100V

The data lines 202 are driven to Vh=100V and the data lines 206 aredriven to Vh=100V. The actuators in the row charge to 200V. The addresslines for the non-selected rows are at Vns=100V.

2. Discharging

The address lines 208 for the non-selected rows are driven to Vns=−100V.

The data lines 202 are driven to V1=−100V. Thus all diodes 200 arenon-conducting, either with 0V across them or a reverse bias of 200V.

The data lines 206 for the actuators to be discharged are driven toVd=−100V. The actuators in those rows are then discharged through thediodes 204. None of the other actuators are affected.

FIG. 22 shows another example in which the switching arrangementcomprises a MIM (metal-insulator-metal) diode 220 in series with theelectroactive polymer actuator 36 between a single column conductor 32and row conductor 30. A MIM diode shows diode characteristics in bothconduction directions with a blocking range at lower voltages.

FIG. 23 shows another example of switching arrangement which comprisesfirst and second transistors 230, 232 in series between a data line 234and a first terminal of the electroactive polymer actuator 36, whereinthe first transistor 230 is switched by a first, column addressing line32 and the second transistor is switched by a second, orthogonal, rowaddressing line 30. A second terminal of the electroactive polymeractuator 36 is connected to a reference potential Vref. A storagecapacitor 38 is also again shown in parallel with the electroactivepolymer actuator. The switching arrangement thus comprises first andsecond transistors in series between a data line and a first terminal ofthe electroactive polymer actuator. The use of two transistors enablesan automatic refreshing scheme to be implemented. This enables theactuation state to be held without re-addressing the array with data.

A driver arrangement is used to deliver first and second drive levels tothe first and second addressing lines, and for delivering data to thedata line.

In order to address the electroactive polymer actuator, both transistorsneed to be addressed simultaneously. Such a circuit allows an automaticrefreshing scheme as e.g. implemented in Random Access Memories (RAM).

In all of the examples above, the driver may apply a data signal in theform of a two level signal (0V and Vdr). This will result in the lowestcost driver IC's as mentioned above. However in alternative embodimentsit may be preferred to also partially actuate the actuators. To enablethis, a data driver with multiple data voltages up to Vdr may be used orelse a PWM drive scheme may be used.

In all of these active matrix addressing schemes, it may be desired toaddress several rows of electroactive polymer actuators at the samemoment, whereby the addressing will proceed even more quickly. This isachieved by applying an addressing voltage to more than one row ofelectroactive polymer actuators at the same time.

As also mentioned above, it may be preferred to invert the polarity ofthe voltage across the electroactive polymer actuators at regularintervals whereby the performance of the device will deteriorate lessthan if inversion is not used. This can be achieved by example bychanging the voltage on the reference electrode in a further round ofaddressing and adapting the driving voltages accordingly.

The electrode arrangement may comprise electrodes on opposite faces ofthe electroactive polymer layer as shown above, for a field drivendevice. These provide a transverse electric field for controlling thethickness of the EAP layer. This in turn causes expansion or contractionof the EAP layer in the plane of the layer.

The electrode arrangement may instead comprise a pair of comb electrodeson one face of the electroactive polymer layer. This provides in-planeelectric field, for directly controlling the dimensions of the layerin-plane.

Materials suitable for the EAP layer are known. Electro-active polymersinclude, but are not limited to, the sub-classes: piezoelectricpolymers, electromechanical polymers, relaxor ferroelectric polymers,electrostrictive polymers, dielectric elastomers, liquid crystalelastomers, conjugated polymers, Ionic Polymer Metal Composites, ionicgels and polymer gels.

The sub-class electrostrictive polymers includes, but is not limited to:Polyvinylidene fluoride (PVDF), Polyvinylidenefluoride-trifluoroethylene (PVDF-TrFE), Polyvinylidenefluoride-trifluoroethylene-chlorofluoroethylene (PVDF-TrFE-CFE),Polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene)(PVDF-TrFE-CTFE), Polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP), polyurethanes or blends thereof.

The sub-class dielectric elastomers includes, but is not limited to:

acrylates, polyurethanes, silicones.

The sub-class conjugated polymers includes, but is not limited to:

polypyrrole, poly-3,4-ethylenedioxythiophene, poly(p-phenylene sulfide),polyanilines.

Additional passive layers may be provided for influencing the behaviorof the EAP layer in response to an applied electric field.

The EAP layer may be sandwiched between electrodes. The electrodes maybe stretchable so that they follow the deformation of the EAP materiallayer. Materials suitable for the electrodes are also known, and may forexample be selected from the group consisting of thin metal films, suchas gold, copper, or aluminum or organic conductors such as carbon black,carbon nanotubes, graphene, poly-aniline (PANI),poly(3,4-ethylenedioxythiophene) (PEDOT), e.g.poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).Metalized polyester films may also be used, such as metalizedpolyethylene terephthalate (PET), for example using an aluminum coating.

The materials for the different layers will be selected for exampletaking account of the elastic moduli (Young's moduli) of the differentlayers.

Additional layers to those discussed above may be used to adapt theelectrical or mechanical behavior of the device, such as additionalpolymer layers.

The EAP devices may be electric field driven devices or ionic devices.Ionic devices may be based on ionic polymer-metal composites (IPMCs) orconjugated polymers. An ionic polymer-metal composite (IPMC) is asynthetic composite nanomaterial that displays artificial musclebehavior under an applied voltage or electric field.

IPMCs are composed of an ionic polymer like Nafion or Flemion whosesurfaces are chemically plated or physically coated with conductors suchas platinum or gold, or carbon-based electrodes. Under an appliedvoltage, ion migration and redistribution due to the imposed voltageacross a strip of IPMCs result in a bending deformation. The polymer isa solvent swollen ion-exchange polymer membrane. The field causescations travel to cathode side together with water. This leads toreorganization of hydrophilic clusters and to polymer expansion. Strainin the cathode area leads to stress in rest of the polymer matrixresulting in bending towards the anode. Reversing the applied voltageinverts the bending.

If the plated electrodes are arranged in a non-symmetric configuration,the imposed voltage can induce all kinds of deformations such astwisting, rolling, torsioning, turning, and non-symmetric bendingdeformation.

The device may be used as a single actuator, or else there may be a lineor array of the devices, for example to provide control of a 2D or 3Dcontour.

The invention can be applied in many EAP applications, includingexamples where a passive matrix array of actuators is of interest.

In many applications the main function of the product relies on the(local) manipulation of human tissue, or the actuation of tissuecontacting interfaces. In such applications EAP actuators provide uniquebenefits mainly because of the small form factor, the flexibility andthe high energy density. Hence EAP's can be easily integrated in soft,3D-shaped and/or miniature products and interfaces. Examples of suchapplications are:

Skin cosmetic treatments such as skin actuation devices in the form ofEAP-based skin patches which apply a constant or cyclic stretch to theskin in order to tension the skin or to reduce wrinkles;

Respiratory devices with a patient interface mask which has an EAP-basedactive cushion or seal, to provide an alternating normal pressure to theskin which reduces or prevents facial red marks;

Electric shavers with an adaptive shaving head. The height of the skincontacting surfaces can be adjusted using EAP actuators in order toinfluence the balance between closeness and irritation;

Oral cleaning devices such as an air floss with a dynamic nozzleactuator to improve the reach of the spray, especially in the spacesbetween the teeth. Alternatively, toothbrushes may be provided withactivated tufts;

Consumer electronics devices or touch panels which provide local hapticfeedback via an array of EAP transducers which is integrated in or nearthe user interface;

Catheters with a steerable tip to enable easy navigation in tortuousblood vessels.

Another category of relevant application which benefits from EAPactuators relates to the modification of light. Optical elements such aslenses, reflective surfaces, gratings etc. can be made adaptive by shapeor position adaptation using EAP actuators. Here the benefits of EAPactuators are for example the lower power consumption.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measured cannot be used to advantage.

Any reference signs in the claims should not be construed as limitingthe scope.

1. A device comprising: an active matrix array of electroactive polymeractuators comprising: a plurality of rows and a plurality of columns; aset of data lines; and a set of addressing lines; a plurality ofswitching arrangements, wherein each switching arrangement is associatedwith each electroactive polymer actuator, wherein each switchingarrangement comprises an input which is connected to a respective dataline and an output which is connected to a first terminal of theassociated electroactive polymer actuator, wherein a second terminal ofeach electroactive polymer actuator is connected to a control line; anda driver circuit arrange to provide drive signals, the drive signalscomprising at least a first drive level, a second drive level, a thirddrive level and a fourth drive level, wherein the first drive level andthe second drive level are for application to the data lines and thethird drive level and the fourth drive level are for application to thecontrol line.
 2. The device as claimed in claim 1, wherein the activematrix array comprises a plurality of sub-arrays and wherein there is ashared control line for each sub-array, wherein each sub-array comprisesa row of electroactive polymer actuators.
 3. The device as claimed inclaim 1, wherein there is a shared control line for all of theelectroactive polymer actuators.
 4. The device as claimed in claim 1,wherein the electroactive polymer actuators have a threshold voltage,wherein the drive signals have a drive voltage, wherein the drivevoltage below the threshold voltage provides minimal actuation, whereinthe drive voltage at a maximum drive voltage provides full actuation. 5.The device as claimed in claim 4, wherein the first and third drivelevels comprise 0V, wherein the fourth drive level comprises a voltagewhich is greater than the voltage of the second drive level.
 6. Thedevice as claimed in claim 5, wherein the voltage of the fourth drivelevel is the maximum drive voltage, wherein the difference between thevoltages of the second and fourth drive levels is equal to or less thanthe threshold voltage.
 7. The device as claimed in claim 5, wherein thevoltage of the fourth drive level is a negative voltage of magnitudeequal to or less than the threshold voltage, wherein the voltage of thesecond drive level is a positive voltage such that difference betweenthe voltages of the second and fourth drive levels is equal to themaximum drive voltage.
 8. The device as claimed in claim 1, wherein theswitching arrangement comprises a transistor.
 9. A method of actuating adevice, wherein the device comprises an active matrix array ofelectroactive polymer actuators, the matrix array comprising a pluralityof rows and a plurality of columns, wherein each electroactive polymeractuator is associated with a switching arrangement, wherein each of theswitching arrangements comprises an input which is connected to arespective data line and an output, wherein the output is connected to afirst terminal of the associated electroactive polymer actuator, whereina second terminal of each electroactive polymer actuator is connected toa control line, the method comprising: setting all the electroactivepolymer actuators to a non-actuated state using a third drive levelwherein the third drive level is provided on the associated controllines; driving all the electroactive polymer actuator actuators towardsa first state by providing a first drive level on the associated datalines and a fourth drive level on the associated control lines; andbefore the electroactive polymer actuators reach the first state drivinga portion of the electroactive polymer actuators to a second state byapplying a second drive level on the associated data lines.
 10. Themethod as claimed in claim 9, wherein the electroactive polymeractuators have a threshold voltage, wherein a drive voltage below thethreshold voltage causes minimal actuation, wherein a maximum drivevoltage causes full actuation.
 11. (canceled)
 12. The method as claimedin claim 10, wherein the first and third drive levels comprise 0V, andwherein the fourth drive level comprises a voltage which is greater thanthe voltage of the second drive level.
 13. The A method as claimed inclaim 12, wherein the voltage of the fourth drive level is the maximumdrive voltage, wherein the difference between the voltages of the secondand fourth drive levels is equal to or less than the threshold voltage.14. (canceled)
 15. The method as claimed in claim 14, wherein thevoltage of the fourth drive level is a negative voltage of magnitudeequal to or less than the threshold voltage, wherein the voltage of thesecond drive level is a positive voltage such that difference betweenthe voltages of the second and fourth drive levels is equal to themaximum drive voltage.
 16. The device as claimed in claim 1, wherein theactive matrix array comprises a plurality of sub-arrays and whereinthere is a shared control line for each sub-array, wherein eachsub-array comprises a column of electroactive polymer actuators.
 17. Thedevice as claimed in claim 8, wherein the transistor is selected fromthe group consisting of a thin film transistor, a polysilicontransistor, an amorphous silicon transistor, or a semiconductor oxidetransistor.